Urban warfare is combat conducted in urban areas such as towns and cities. Urban combat is very different from combat in the open at both the operational and tactical level. Complicating factors in urban warfare include the presence of civilians and the complexity of the urban terrain.

Some civilians may be difficult to distinguish from combatants such as armed militias and gangs, and particularly individuals who are simply trying to protect their homes from attackers. Tactics are complicated by a three-dimensional environment, limited fields of view and fire because of buildings, enhanced concealment and cover for defenders, below ground infrastructure, and the ease of placement of booby traps and snipers.^{[citation needed]}

The United States military term for urban warfare is UO an abbreviation for urban operations. The previously used US military term MOUT, an abbreviation for military operations in urban terrain, has been replaced by UO, although the term MOUT Site is still in use.

The British military terms are OBUA (operations in built-up areas), FIBUA (fighting in built-up areas), or sometimes (colloquially) FISH (fighting in someone's house), or FISH and CHIPS (fighting in someone's house and causing havoc in people's streets)..